With the evolution of the Internet environment and increases in data centers based on the widespread use of cloud computing and the like, the volume of generated information is rapidly increasing in recent years. It is safe to say that magnetic storage apparatuses, such as the hard disk drive (HDD) which offers the highest recording density and superior bit cost, will play a dominant role in the era of “Big Data” storage. Thus, it is necessary to increase the capacity of magnetic storage apparatus and achieve higher recording density to support the increase in storage capacity.
As a new magnetic recording technology for achieving higher recording density, the microwave assisted magnetic recording (MAMR) system has been proposed whereby magnetic recording in a perpendicular magnetic recording medium having high magnetic anisotropy is performed by applying a microwave-band high frequency magnetic field to the magnetic recording medium to excite precession of the medium magnetization, thus lowering the switching field. In recent years, a practical spin torque type high frequency oscillating element (Spin Torque Oscillator: STO) having a microstructure has been proposed, such as disclosed in Patent Document 1. The spin torque oscillator, to which spintronics technology is applied, generates a high frequency magnetic field by causing a high-speed rotation of spin in a high frequency magnetic field generation layer (Field Generation Layer: FGL) by the spin torque of spin injected from a spin injection layer driven by a direct-current power supply. The spin injection layer and the FGL are stacked via, for example, an electrically conductive intermediate layer of noble metal, such as Cu, Pt, Au, Ag, Pd, and Ru, or non-magnetic transition metal, such as Cr, Rh, Mo, or W (Patent Document 2). Patent Document 3 discloses a spintronics element for microwave assisted recording in which, as an underlayer for the spin injection layer of a [Co/Ni] magnetic super-lattice in which Co and Ni layers with film thicknesses on the order of several atomic layers are stacked on a substrate such that the thickness of the Ni layer is greater than the thickness of the Co layer, a composite seed layer including a lower Ta layer and a metal layer formed thereon having a fcc [111] crystalline structure or a hcp [001] crystalline structure is used.
Further, Patent Document 4 discloses a method for more efficiency inducing magnetization reversal by causing a high frequency magnetic field oscillating element to generate a high frequency magnetic field (circularly polarized magnetic field) that rotates in the same direction as the precession direction of magnetization of the magnetic recording medium in which magnetization reversal is desired, in accordance with the polarity of the recording field. Thus, research and development for practical application of the microwave assisted magnetic recording system is gaining momentum.
For the read element too, an STO sensor has been proposed whereby, based on the phenomenon in which the oscillation frequency due to spin torque effect varies depending on the magnitude of an external magnetic field, the recording magnetization state from the magnetic recording medium is detected by detecting a frequency change while the decrease in S/N due to magnetic thermal fluctuation noise caused by miniaturization is lowered. For example, Patent Document 5 proposes, in an STO sensor including a high frequency oscillating element and a second oscillating element disposed in the vicinity thereof that oscillates at a frequency close to the oscillation frequency of the high frequency oscillating element for locking the oscillation, a read element that acquires a high frequency oscillation signal generated across the ends of the high frequency oscillating element and the second oscillating element when energized perpendicularly to the film plane of the high frequency oscillation layer. Patent Document 6 proposes an STO sensor in which a magnetization free layer (free layer) is stacked via a non-magnetic layer. Patent Document 7 proposes an STO sensor in which two layers of magnetic films having longitudinal magnetic easy axes which are perpendicular to each other are stacked via a non-magnetic intermediate layer.